In recent years, when an optical modulator is evaluated, a characteristic called jitter indicating a periodic variation in an optical signal obtained during the driving of the optical modulator has drawn attention.
Jitter is an index indicating a periodic variation in the optical signal. As shown in FIG. 1, optical eye pattern waveforms are added up, and the jitter is defined as the width of an intersection of signals.
In order to reduce the jitter of the optical signal obtained during the driving of the optical modulator, it is necessary to improve the following characteristics of an optical modulator or a driver that controls the driving of the optical modulator.
(1) Driver
The driver has frequency characteristics in which gain is flat from a low frequency band to a high frequency band such that an input electric signal is amplified without being deteriorated.
(2) Optical Modulator
The optical modulator has frequency characteristics in which the frequency of an electric/optical conversion response is flat from a low frequency band to a high frequency band such that an input electric signal is converted into an optical signal without being deteriorated.
As described above, when the frequency characteristics of the driver and the optical modulator are infinitely flat (when there is no frequency dependence), the above-mentioned jitter does not occur. However, actually, the frequency characteristics of both the driver and the optical modulator are not flat in a low frequency band, or the frequency characteristics thereof in the high frequency band tend to deteriorate to the lower right side, which results in jitter. In particular, in recent years, a traveling-wave-type optical modulator having a transfer rate of more than 40 Gbps has been used, and the occurrence of the jitter has become an important problem.
In order to reduce the jitter, it is necessary to improve the characteristics of each of the driver and the optical modulator. However, it is very difficult to improve the characteristics of each of the driver and the optical modulator and make the frequency characteristics thereof flat.
For example, since the frequency characteristics of the driver are determined at the design stage, it is difficult to improve the frequency characteristics during or after the manufacture of the driver.
Patent Document 1 discloses a technique that adjusts the impedance of an end portion of a modulating electrode of an optical modulator to improve the frequency characteristics of the optical modulator. However, in this structure, it is difficult to make the frequency characteristics flat up to a high frequency band that enables 40 Gbps transmission.
Only the adjustment of the impedance of the end portion disclosed in Patent Document 1 is insufficient to change a frequency to be adjusted, among the frequency characteristics of the electric/optical conversion response of a traveling-wave-type optical modulator.    [Patent Document 1] Japanese Patent No. 3088988
Even when each of the driver and the optical modulator has good frequency characteristics, jitter may occur when the driver and the optical modulator are combined with each other. Therefore, compatibility between the driver and the optical modulator also needs to be considered.
Therefore, a complicated operation of selecting an optical modulator to be combined with the driver used or selecting a driver to be combined with the optical modulator used is needed in order to obtain good optical transmission characteristics with a small jitter.
In order to solve the above-mentioned problems, the inventors have proposed a traveling-wave-type optical modulator capable of preventing the occurrence of jitter when a driver is used to drive an optical modulator and a method of adjusting the same, particularly, a traveling-wave-type optical modulator capable of improving the flexibility of a combination of a driver and a traveling-wave-type optical modulator and effectively preventing jitter even after the combination, and a method of adjusting the same, which is disclosed in the following Patent Document 2.    [Patent Document 2] International Publication No. WO 2005/096077
In Patent Document 2, a traveling-wave-type optical modulator includes a substrate that has an electro-optical effect, an optical waveguide that is formed on the substrate, and a modulating electrode that controls the modulation of a light wave traveling through the optical waveguide. A driver that controls the driving of the traveling-wave-type optical modulator is connected to the traveling-wave-type optical modulator, and the frequency characteristics of the electric/optical conversion response of the traveling-wave-type optical modulator are adjusted such that the frequency characteristics of the gain of the driver are corrected.
In particular, when the traveling-wave-type optical modulator has a termination resistor, the adjustment of the frequency characteristics of the electric/optical conversion response can be performed by adjusting at least one of the impedance of the modulating electrode and the impedance of the termination resistor. In addition, the adjustment of the frequency characteristics of the electric/optical conversion response at a specific frequency can be performed by adjusting at least one of the length of the modulating electrode and the effective refractive index of a microwave applied to the modulating electrode. Further, the adjustment of the frequency characteristics of the electric/optical conversion response are performed by adjusting the frequency characteristics of the electric/optical conversion response of an optical signal emitted from the traveling-wave-type optical modulator in the range of 300 kHz to 3 GHz with respect to an electric signal input to the driver when the driver drives the traveling-wave-type optical modulator to be approximately flat.
However, in order to reduce jitter and obtain better optical transmission characteristics, it is necessary to adjust the frequency characteristics of the electric/optical conversion response of the traveling-wave-type optical modulator to which the technique disclosed in Patent Document 2 is applied in a wide frequency range including a high frequency band.
Further, in order to obtain good frequency characteristics, it is indispensable to improve the characteristics of electric signals input to the traveling-wave-type optical modulator. In particular, it is necessary to consider all combinations of a signal source and electronic parts or cables through which electric signals pass.